1. Field of the Invention
The present invention relates to a sheet feeder and a copying machine including the same and, more particularly, relates to a sheet feeder having a double-feed detecting function for detecting sheets being transferred while being overlapped with one another.
2. Description of Related Art
As a prior art relating to the present invention, there have been known a double-feed detecting apparatus which include a sheet-length detection device for measuring sheet-length being transferred in a transfer direction and a sheet-thickness detection device for measuring sheet-thickness, wherein the sheet thickness detection device measures the sheet-thickness in vicinity of a middle of the sheet-length measured by the sheet-length detection device, thereby enabling certainly detecting double feed of sheets, even though the sheets are misaligned in the feeding direction and overlapped with one another (see for example, Japanese Unexamined Patent Publication No. HEI 9(1997)-142699).
In recent years, among image forming apparatus such as copying machines, image forming apparatus which include an auto document feeder (hereinafter, abbreviated to an “ADF”) for automatically feeding sheets to an image reading portion thereof have become a mainstream.
Such an ADF transfers sheets, one by one, from a stack of sheets placed on a sheet tray to a sheet transfer path and further transfers them to the image reading portion of the image forming apparatus.
Among these ADFs, there are some ADFs which have a function of detecting double feed of sheets, in order to prevent sheets from being transferred to the sheet transfer path at double-feed states where the sheets are overlapped with one another to cause incorrect reading of images intended to be read, thus resulting in improper printing inconsistent with user's requests and malfunctions such as page dropouts.
Exemplary means for detecting double feed of sheets are methods which utilize an optical sensor or an ultrasonic wave sensor provided in the sheet transfer path.
With such a method which utilizes an optical sensor, change of a quantity of light generated from a light generating device is detected with the optical sensor when the light passes through sheets, and a comparison is made between the light quantity change and a predetermined threshold value to determine whether sheets are being transferred normally one by one or being transferred at a double-feed state.
On the other hand, with such a method which utilizes an ultrasonic wave sensor, utilizing a fact that an ultrasonic wave generated from a wave generating device is attenuated by sheets when it is passed through the sheets, a comparison is made between a degree of attenuation of the ultrasonic wave received by a wave receiving device and a predetermined threshold value to determine whether the sheets are transferred normally one by one or transferred at a double-feed state.
Further, there is a possibility that the stack of sheets placed on the sheet tray of the ADF contains sheets on which labels are partially attached or sheets on which cutout paper pieces are partially attached.
In this case, with the method utilizing an optical sensor, the quantity of light passed through sheets is decreased to above the predetermined threshold value at portions of sheets to which such labels or paper pieces are attached, thereby inducing false detection of double feed, even though sheets are normally transferred one by one.
Further, even with the method utilizing an ultrasonic wave sensor, similarly to with the method utilizing an optical sensor, the degree of attenuation of the ultrasonic wave is increased to above the predetermined threshold value at portions of sheets to which labels or paper pieces are attached, thereby inducing false detection of double feed.